Catalytic converters are devices used to reduce pollutants from motor vehicle exhaust gases. Catalysts promote chemical reactions that convert pollutants such as carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NOx) into carbon dioxide, water and nitrogen.
Known methods for catalyst monitoring include monitoring based on direct HC or NOx sensor measurement, monitoring based on temperature measurement, and monitoring based on the converter's oxygen storage capacity with dual oxygen sensors placed upstream and downstream of the converter. Currently available HC and NOx sensors are expensive and less stable than other available sensors, such as oxygen sensors or thermocouples.
To date, temperature measurement approaches are prone to noise sources, such as ambient temperature and wind conditions. Furthermore, different starting up processes by different drivers or the same driver at different occasions will force the engine to run in different patterns which, in turn, will significantly change the time history of the exhaust gas temperature and flow rate.
Unacceptable aging of a catalyst may be detected based on oxygen storage capacity by first biasing the oxygen level in the exhaust system to a rich or lean amount. Counts of rich/lean and lean/rich transitions of upstream and downstream oxygen sensors are accumulated, and a ratio of the counts is then used in a so-called frequency ratio test to determine the amount of untreated exhaust gas breaking through the catalyst. While this method is generally effective, it suffers drawbacks of being intrusive to the engine fueling control system and thereby increasing pollutant emissions during execution of the method of monitoring. Since pollutant emissions are increased while this method executes, execution time must be minimized, often leading to incorrect monitoring results. Additionally, the oxygen storage capacity monitoring approach is prone to noise disturbances.
In a copending, commonly assigned U.S. patent application Ser. No. 10/348,553, filed Jan. 21, 2003, an unintrusive catalyst monitoring method is described. This method generates a catalyst performance index based on behavior of both upstream and downstream oxygen sensors placed at the catalyst input and output, but without looking into the catalyst's internal characteristics.
Therefore, there is a need for an unintrusive catalyst monitoring approach using modeling of the dynamic behavior of the catalytic converter for improved, noise-resistant detection of an unacceptably aged catalyst.